Backup processing of business data to a physical tape volume is performed on a routine basis. To shorten the backup processing, there is known a virtual tape device that uses a RAID (Redundant Arrays of Inexpensive Disks) device, which is a magnetic disk device of a RAID structure, as a cache of data to be stored in a physical tape volume.
As a prior art relating to the present invention, the following documents have been disclosed.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 7-272409    [Patent Document 2] Japanese Laid-Open Patent Publication No. 9-274540